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          什么是倒排索引?
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        <h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>在学习 ES 的过程中，简单的介绍下倒排索引是怎么回事，对倒排索引有一个宏观的感受。</p>
<h1 id="正排索引"><a href="#正排索引" class="headerlink" title="正排索引"></a>正排索引</h1><p>在讲述倒排索引之前先介绍下正排索引。正排索引就是如下表形式：</p>
<table>
<thead>
<tr>
<th>文档id</th>
<th align="left">文档内容</th>
</tr>
</thead>
<tbody><tr>
<td>1</td>
<td align="left">什么是正排索引</td>
</tr>
<tr>
<td>2</td>
<td align="left">什么是倒排索引</td>
</tr>
<tr>
<td>3</td>
<td align="left">正排索引和倒排索引</td>
</tr>
</tbody></table>
<span id="more"></span>

<h1 id="倒排索引"><a href="#倒排索引" class="headerlink" title="倒排索引"></a>倒排索引</h1><p>倒排索引就是对上表进行转换，最简单的倒排索引如下表所示：</p>
<table>
<thead>
<tr>
<th>单词</th>
<th>文档ids</th>
</tr>
</thead>
<tbody><tr>
<td>什么</td>
<td>1,2</td>
</tr>
<tr>
<td>是</td>
<td>1,2</td>
</tr>
<tr>
<td>正排</td>
<td>1,3</td>
</tr>
<tr>
<td>倒排</td>
<td>2,3</td>
</tr>
<tr>
<td>索引</td>
<td>1,2,3</td>
</tr>
<tr>
<td>和</td>
<td>3</td>
</tr>
</tbody></table>
<h2 id="词频（TF）和位置（POS）"><a href="#词频（TF）和位置（POS）" class="headerlink" title="词频（TF）和位置（POS）"></a>词频（TF）和位置（POS）</h2><p>上表所示的倒排索引之所以是最简单的，是因为这个索引系统只记载了哪些文档包含某个单词。实用的倒排索引还可以记载更多的信息，如下表第3列的<strong>词频</strong>和<strong>位置</strong>。</p>
<p><strong>TF</strong>：单词在某个文档中出现的次数<br><strong>POS</strong>：单词在文档中出现的位置</p>
<table>
<thead>
<tr>
<th align="left">单词</th>
<th>文档ids</th>
<th>文档id:词频TF:&lt;位置POS&gt;</th>
</tr>
</thead>
<tbody><tr>
<td align="left">什么</td>
<td>1,2</td>
<td>1:1:&lt;1&gt;,2:1:&lt;1&gt;</td>
</tr>
<tr>
<td align="left">是</td>
<td>1,2</td>
<td>1:1:&lt;2&gt;,2:1:&lt;2&gt;</td>
</tr>
<tr>
<td align="left">正排</td>
<td>1,3</td>
<td>1:1:&lt;3&gt;,3:1:&lt;0&gt;</td>
</tr>
<tr>
<td align="left">倒排</td>
<td>2,3</td>
<td>2:1:&lt;3&gt;,3:1:&lt;5&gt;</td>
</tr>
<tr>
<td align="left">索引</td>
<td>1,2,3</td>
<td>1:1:&lt;5&gt;,2:1:&lt;5&gt;,3:2:&lt;2,7&gt;</td>
</tr>
<tr>
<td align="left">和</td>
<td>3</td>
<td>3:1:&lt;4&gt;</td>
</tr>
</tbody></table>
<p>以“索引”这个单词为例，“3:2:&lt;2,7&gt;”：表示在文档id是3的文档中出现2次，在文档中的位置分别是2和7。</p>
<h1 id="参考资源"><a href="#参考资源" class="headerlink" title="参考资源"></a>参考资源</h1><p><a target="_blank" rel="noopener" href="https://www.cnblogs.com/ottll/p/9470732.html">https://www.cnblogs.com/ottll/p/9470732.html</a></p>

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